Hi. When I am riding my Surly, and am at the higher gear...2-7 or 2-8 or 2-9, the chain doesn't seem to want to stay in gear.....It hops down, it hops back up....totally annoying and affects my rhythm......SO I stay down at 2.6 or go to 3.1............Ni sure of the terminology, so I hope someone understands what I am saying. Can someone explain what is going on and what I might need to do ? Thanks